Method of bleaching pulp



Patented Feb. 11, 1936 METHOD OF BLEACHING PULP Arno J. Luth, PortEdwards, Richard A. Nugent,

Nekoosa, and Neil H. Christian, Port Edwards,

Wis.,

assignors to Nekoosa-Edwards Paper Company, Port Edwards; Wis., a.corporation of Wisconsin No Drawing. Application October 18, 1934,Serial No. 748,923

7 Claims.

This invention relates to a method of bleaching pulp and moreparticularly to a method of bleaching pulp prepared by the sulphateprocess, or by some similar alkaline process using a 5 sulphide as anactive cooking ingredient.

The coloring matter in sulphate pulp is largely composed of complexphlobotannin bodies containing sulphur in their constitutions. While theexact chemical constitutions of the phlobotannin 10 bodies are not knownwith certainty, certain facts have been observed with respect to thephlobotannin bodies. In general, the sulphur content of the complexphlobotannin bodies increases as the sulfldity of the cooking liquorincreases. As

15 the concentration of sulphur in the cooking liquor increases, and asthe cooking conditions are made more drastic by the use of highertemperatures, the substitution of sulphur in the coloring matter, whichhas been identified as sul- 20 phur derivatives of phlobotannin bodies,increases and makes the coloring matter darker brown or black.

The bleachability of sulphate pulp is quite largelydependent upon theconcentration of 5 phlobotannin bodies in the pulp. Furthermore, thedifliculty in bleaching sulphate pulp increases as the sulphur contentof the phlobotannin bodies increases. The sulphuretted phlobotanninbodies may be considered as sulphur' dyes, which must 30 be removed ,oraltered before bleaching, if the bleach requirements are to be reducedand a white pulp is to be obtained.

The coloring matter comprising these sulphur derivatives of phlobotanninbodies is acidic in 35 nature. In alkaline solution, the coloring matteris a darker red brown, while in 'acid solution it is a lighter brownishyellow. The coloring matter is darkest in'color at pH 10.5 or above andlightest in color at pH 2.8 or below. The greatest 40 percentage ofcolor change occurs at pH 6.9

It is interesting to note that the coloring matter in sulphate or kraftpulp does not obtain its characteristic brown color until it comes incon- 45 tact with oxygen, whichmay be introduced in the wash liquors, inthe water used, or from the air. Before contact with oxygen, thecoloring matter is a bright yellow, more or less-dark depending upon thesulphur content.

50 In "order to improve the bleachability of sulphate pulp and obtain ableached pulp of the desired degree of whiteness, and preferably a whitepulp having a bluish cast, we have found that a portion of thephlobotannin bodies can be.

55 removed prior to bleaching by the use of a strong caustic solution,as described in our copending application, Method of treating pulp,filed 0ctober 18, 1934, Serial No. 748,921. It. has also been found thatupon chlorination, the coloring matter is rendered soluble in causticsoda, but not in lime, so that if the bleaching liquor is maintainedalkaline, the coloring matters can be washed out.

We have now found that the addition of oxygen liberating compounds, suchas alkali metal per- 10 oxides, to the caustic soda 'solution used inthe treatment of the sulphate pulp prior to bleaching makes it possibleto obtain a bleached sulphate pulp of a high degree of whiteness, and ofa white color that is on the bluish rather than the yellowish side. Asmall percentage of a peroxide of'an alkali metal is added to thecaustic soda solution that is used for treatment of the sulphate pulp inthe diffuser and is also added to the caustic soda solution used fortreatment of the pulp between the chlorination and bleaching steps.

By the method of our invention, a bleached pulp is produced that can besatisfactorily used in the manufacture of high grade bond, ledgerandwriting papers. The bleached pulp is of an exceptionally high purity,having a high alpha cellulose content and a low copper number. It alsohas very desirable paper making characteristics, in that it resists themechanical action. of heaters and Jordans to such an extent as to becapable of withstanding longer beating times and thus of requiring ahigher degree of hydration.

A pulp having these improved paper making characteristics can bemanufactured into high grade papers having surface characteristicssimilar to those of rag paper.

It is therefore an important object of this invention to provide amethod of bleaching sulphate pulp to obtain a bleached sulphate pulp ofa high degree of whiteness and of improved characteristics, capable ofbeing made into high grade papers possessing surface characteristicssimilar to those of rag papers.

It is a further important object of this invention to provide a methodof bleaching sulphate pulp wherein the phlobotannin bodies are subjectedto oxygen in nascent form, and are probably converted, in part, into ablue coloring gester in which the wood is cooked, into a diffuser, wherethe pulp is washed to remove the cooking liquor, or black liquor, as itis commonly bluish shade.

' The caustic soda solution used in treating the and an alkali metalperoxide, before the color bodies have had an opportunity to becomeoxidized, the color bodies are quite largely dissolved or kept insolution and subsequently removed. The action of the alkali metalperoxides appears to be one that results in the conversion of part ofthe coloring matter into color bodies of a sulphate pulp in the diffuseris preferably one that has been prepared by dissolving caustic soda pulpundergoing treatment.

in fresh water, although so-called white liquor, as it is known in thesulphate process, which is a strongly alkaline liquor produced bycausticizing green liquor, may be used. The amount of caustic sodasolution used in our method will vary considerably, depending upon thetime of contact of the caustic liquor with the pulp, the concentrationof the caustic liquor or solution, and the temperature of the mass inthe diffuser. Ordinarily, however, between 2.5 and 15% of caustic soda(NaOH) by weight of the air dry pulp undergoing treatment constitutesthe most satisfactory operating range. A caustic solution of aconcentration of approximately 50% of NaOH by weight of the solution maybe used. Into this caustic solution is incorporated by dissolving aquantity of sodium peroxide equivalent to 0.5% or over of the air dryweight of the Other peroxides of alkali. metals or perborates may beused in place of sodium peroxide.

As previously stated, the treatment of the pulp in the diffuser withcaustic solution containing sodium peroxide is started as soon aspossible after the pulp has been blown into the diffuser, so as to avoidany substantial oxidation of the phlobotannin bodies. A caustic solutionof the concentration above indicated is introduced into the space abovethe pulp in the diffuser and is gradually forced downwardly through themass of pulp and cooking liquor, displacing the cooking liquor ahead ofit. After all of the caustic solution has been introduced into thediffuser, it is followed by weak liquor from the process, or by washwater, introduced under suiiicient pressure, say about 15 pounds per sq.in., to displace the body of caustic solution ahead of it at the desiredrate, such that the operation may be completed in from 12 to 16 hours.The action of the caustic soda solution isto dissolve the phlobotanninbodies and sulphur derivatives thereof, and these bodies and derivativesare largely removed by the caustic soda solution and by the weak liquoror wash liquor following the caustic soda treatment.

After the pulp in the diffuser has been thoroughly washed, it issubjected to the usual screening operation, being first passed through aknotter and then through screens to prepare the pulp for bleaching. Thebleaching process is preferably of a multi-stage type, including atleast one treatment with gaseous chlorine at a comparatively low pulpdensity and a subsequent, and preferably final, treatment, also at lowdensity, with bleaching solutions containing calcium hypochlorite and ofmaintained caustic alkalinity, as taught in the Wells Patent No.1,883,193. Multi-stage bleaching operations of this type are not inthemselves new, but the method of our invention includes an intermediatetreatment of the pulp with a caustic alkali solution containing analkali metal peroxide to aid in the removal or conversion of the colorbodies. This treatment with caustic alkali and alkali metal peroxidebetween the chlorinating and calcium hypochlorite bleaching processsupplements the treatment of the pulp in the diffuser in the manneralready described, and is apparently necessary to obtain the bestresults from the diffuser treatment.

The first chlorination step may be carried out with the use of gaseouschlorine and at a consistency of between 3 and 5%. Chlorine isintroduced into the aqueous suspension of pulp in any suitable manner,as by bubbling chlorine gas into a downwardly flowing column of pulp ata rate such that the bubbles of chlorine gas do not rise substantiallyin the column before being absorbed. Sufficient chlorine is introducedin this manner to equal about 45% of the chlorine required for completebleaching of the pulp. The temperature of the pulp mass duringchlorination is preferably maintained at between 70 and 75 F. After thepreliminary bleaching step just described, the pulp is washed with acaustic soda solution of 40 to 50% concentration, which is used in suchan amount as to provide between 0.5 and 3% of caustic soda by weight of,the air dry pulp undergoing treatment. The caustic solution ispreferably added continuously to the stock while it is being moved alongin the system. After the addition of the caustic solution, the stock ispassed through a reacting tower, or vessel, wherein it is allowed toremain for to 20 minutes to permit the reaction between caustic soda andany chlorinated bodies, such as lignin and ligno celluloses, to takeplace. Chlorination of the phlobot'annins renders them soluble in strongalkalies, such as caustic soda, have been oxidized, whereas they arerendered insoluble and precipitated by the hydroxides of alkaline earthmetals, such as lime.

The stock is thereafter again washed and concentrated to a consistencyof between 12 and 20%, as by means of a screw press. To the concentratedmass of stock there is added a quantity of caustic soda solution in anamount equivalent to about 5% of caustic soda (NaOH) on the air dryweight of the pulp present. Thiscaustic soda solution contains an amountof sodium peroxide equivalent to about 0.5% or over by weight of the airdry pulp undergoing treatment. The thus treated stock is heated to atemperature of between 100 and 120 F., preferably about 100 F., andmaintained at that temperature for approximately one hour. During thistreatment, the strong caustic solution dissolves any remainingimpurities, such as lignin and phlobotannin bodies that have beenchlorinated during the bleaching operation. It also appears that anyremaining coloring matter is converted by the action of the sodiumperoxide into a bluish color body.

Ater this treatment with caustic soda and sodium peroxide, the pulp isagain washed and then after they.

bleached at low density, say from 3 to 6%, with an aqueous bleachingsolution of calcium hypochlorite. The amount of bleaching solution addedis that calculated to complete the bleaching of the pulp. Bleaching iscarried out at a temperature of between 100 and 120 F., preferably atabout 115 F. During this final bleaching step, the stock is maintainedalkaline to phenolphthalein by the addition of caustic soda ifnecessary.

When the stock has been completely bleached, it is removed from thebleacher and washed. During washing, a dilute mineral acid, such assulphuric acid, may be added as a neutralizing agent to remove the lasttraces of alkali in the stock.

Bleached sulphate pulp made in accordance with the method abovedescribed is of a high degree of purity and is more on the bluish shadeof white than bleached sulphate pulp produced by methods not employingan alkaline metal peroxide or other oxygen liberating compound in thetreatment of the pulp.

We are aware that numerous details of the process may be varied througha wide range without departing from the principles of this invention,and we, therefore, do not purpose limiting the patent granted hereonotherwise than necessitated by the prior art.

We claim as our invention:

1. The method of obtaining bleached pulp of improved characteristicsfrom pulp prepared by the use of analkaline cooking liquor containing'an active sulphide ingredient, which comprises treating the pulp beforeit is washed and before it is exposed to the oxidizing influences ofdissolved and atmospheric oxygen with a caustic alkali solutioncontaining an oxygen liberating alkali metal compound, subjecting thetreated pulp after washing to a multi-stage bleaching operation andtreating said pulp between successive bleaching steps with a causticalkali solution also containing an oxygen liberating alkali metalcompound.

2. The method of obtaining bleached pulp of improved characteristicsfrom pulp prepared by the use of an alkaline cooking liquor containingan active sulphide ingredient, which comprises treating the pulp in aclosed vessel under pressure before the pulp has been exposed to theoxidizing influences of dissolved and atmospheric oxygen with a freshcaustic alkali solution containing sodium peroxide, subjecting thetreated pulp after washing to a multi-stage bleaching operation andtreating said pulp between successive bleaching steps with a causticalkali solution containing sodium peroxide.

3. In a method of obtaining bleached pulp of improved characteristicsfrom pulp prepared by the use of an alkaline cooking liquor containingan active sulphide ingredient, which comprises treating the pulp in aclosed vessel under pressure before being exposed to the oxidizinginfluences of dissolved and atmospheric oxygen with a fresh causticalkali solution containing sodium peroxide, the sodium peroxide beingpresent in an amount equivalent to 0.5% and over on the air dry weightof the pulp undergoing treatment, subjecting the treated pulp afterwashing to a multi-stage bleaching operation and treating said pulpbetween successive bleaching steps with an alkaline solution containingsodium peroxide in an amount equivalent to 0.5% and over on the air dryweight of the pulp undergoing treatment.

4. In the process of obtaining a bleached sulphate pulp of improvedcharacteristics, the step of subjecting freshly blown pulp before it-isexposed to the oxidizing influences of dissolved and atmospheric oxygento the actlonof a solution of caustic soda containing an oxygenliberating alkali metal compound.

5. In the process of obtaining a bleached sulphate pulp of improvedcharacteristics, the step of subjecting freshly blown pulp before it isexposed to the oxidizing influences of dissolved and atmospheric oxygento the action of a solution of caustic soda and sodium peroxide.

6. In the process of obtaining a bleached sulphate pulp of improvedcharacteristics, the step c: subjecting freshly blown pulp before it isexposed to the oxidizing influences of dissolved and atmospheric oxygento the action of an aqueous solution of caustic soda and sodiumperoxide, the amount of caustic soda being equivalent to 2.5 to 15% ofNaOH on the air dry weight of the pulp and the amount of sodium peroxidebeing equivalent to 0.5% and higher on the air dry weight of the pulp.

'7. In the process of obtaining a bleached sulphate pulp of improvedcharacteristics, the steps of subjecting freshly blown pulp before it isexposed to the oxidizing influences of dissolved and atmospheric oxygento the action of an aqueous solution of caustic soda and sodium peroxideand subsequently between successive bleaching operations againsubjecting the pulp to the action of a similar solution containingcaustic soda and sodium peroxide.

ARNO J. LU'IfH. RICHARD A. NUGEN'I. NEIL H. CHRISTIAN.

